With the technological development in recent years of a magnetic bearing that suspends a rotor without contact, magnetic bearings are being used for various types of bearings. A magnetic bearing utilizes an electromagnet and requires a large amount of electrical current to levitate a rotor, resulting in a large power consumption. Therefore, in order to obtain a large amount of magnetic force with a small amount of current, the space between the rotor and stator is required to be narrow. Additionally, a high work accuracy is required. For those reasons, a hybrid magnetic bearing utilizing the bias flux of a permanent magnet is used.
One basic configuration of the hybrid magnetic bearing has a magnetized permanent magnet sandwiched in the axial direction between two radial magnetic bearings disposed at a distance from each other in the rotation axis direction of the rotor, and one of the radial bearings is made to be the positive pole. Another radial bearing is bias-magnetized to have the negative pole. By increasing the bias flux generated in the above manner in one of the radial directions and decreasing the bias flux in the other direction with a magnetizing coil, radial attraction is controlled.
A single radial magnetic bearing modified to be a hybrid type for the purpose of size reduction is also known. A magnetic bearing disclosed in Patent Document 1 has a first magnetic pole face that is one of the outer peripheral faces of a group of magnetic members in a ring-shaped rotor that consists of a permanent magnet magnetized in an axial direction sandwiched between the magnetic members from the axial direction. Another outer peripheral face of the magnetic members is the second magnetic pole face. The periphery of the rotor has a stator with four electromagnets, and each of the four electromagnets is mounted oppositely to the magnetic pole face of the rotor. It has been proposed to have passive magnetic support of the axial direction and to allow a tilting in the rotor by having such a configuration that connects the rotor and the stator by strong magnetic force.
Patent Document 2 proposes a magnetic bearing having a configuration of a magnetic bearing in which a rotor in the center has angulated U-shaped electromagnets arranged around its circumference at regular intervals, and the electromagnets are connected so that the magnetized directions are arranged alternately, said arranging being performed by a permanent magnet magnetized in the circumferential direction.
Patent Document 3 proposes a magnetic bearing having a configuration in which a permanent magnet magnetized in a radial direction is placed so as to cover the end of a stator having plural salient poles, providing a bias flux.
However, the magnetic bearings disclosed in Patent Documents 1 and 3 are of configurations in which a permanent magnet is arranged on a magnetic path where the flux of a control coil travels, and because the permanent magnet is a gap for the control flux, there is a limit to the amount that the control force can be increased by increasing the bias flux and increasing the thickness of the permanent magnet.
Although the magnetic bearing disclosed in Patent Document 2 can generate a strong control force if the control is in the X axis direction only or in the Y axis direction only, the simultaneous control of the X axis and Y axis causes a problem such that the strong control force cannot be generated because the control flux is being interfered with.
Because the magnetic bearing suspends a rotor without contact, the magnetic levitation is normally unstable and therefore needs to be stabilized by detection of the position of levitation and by feedback control. As a sensor detecting the position of levitation, an eddy-current sensor and an inductor sensor are used; however, these are generally expensive. In addition, if the magnetic bearing and the sensor have to be separately placed, the stability range of the feedback system will be small and there might be difficulty in stabilization. In particular, magnetic bearings for ultra-small rotors have been sought in recent years, and limitations in the sensor placement space are a problem in reducing the size.
There exists position detection of the magnetic bearing using a self-sensing technology that utilizes the electromagnet of the magnetic bearing as a sensor. When the position of a rotor changes, the magnetic pole inductance changes as the distance (gap) from the magnetic pole of the magnetic bearing to the rotor changes. By detecting the change in the inductance using any appropriate method, the gap can be estimated. The methods that have been attempted include a method of estimating displacement of a rotor on the basis of the electrical current or voltage of the high-frequency component by overlapping the magnetizing coil of an electromagnet with a high-frequency signal, and a method of establishing a mathematical model of the rotor/magnetic bearing system and generating an observation of the displacement estimation on the basis of the model. However, the magnetic bearing employing the self-sensing has a problem in that estimation accuracy of the position is lower than that of a method that employs a displacement sensor separately.
Patent Document 1:
Japanese Patent Application Publication No. 2005-121157
Patent Document 2:
Japanese Patent Application Publication No. 2001-41238
Patent Document 3:
Japanese Patent Application Publication No. H11-101234